The present invention relates to an exposure apparatus for exposing an object, such as a single crystal substrate and a glass plate for a liquid crystal display (“LCD”). The present invention is especially suitable for an exposure apparatus that uses ultraviolet (“UV”) and extreme ultraviolet (“EUV”) light as an exposure light source.
Along with the recent demand on smaller and lower profile electronic devices, a miniaturization of semiconductor devices to be mounted onto these electronic devices have been increasingly demanded. For example, a design rule for a mask pattern requires that an image with a size of a line and space (L & S) of less than 0.1 μm be extensively formed, and predictably, it will further move to a formation of circuit patterns of less than 80 nm in the future. L & S denotes an image projected to a wafer in exposure with equal line and space widths, and serves as an index of exposure resolution.
A projection exposure apparatus, which is a typical exposure apparatus for fabricating semiconductor devices, includes a projection optical system that projects and exposes a pattern formed on a mask or a reticle (which are used interchangeably in the present application) onto a wafer. Resolution R of a projection exposure apparatus (i.e., a minimum size which enables a precise transfer of an image) can be given by using a light-source wavelength λ and the numerical aperture (NA) of the projection optical system as in the following equation:
                    R        =                              k            1                    ×                      λ            NA                                              (        1        )            
As the shorter the wavelength becomes and the higher the NA increases, the better the resolution becomes. The recent trend has required that the resolution be a smaller value; however it is difficult to meet this requirement using only the increased NA, and the improved resolution expects use of a shortened wavelength. Exposure light sources have currently been in transition from KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm) to F2 excimer laser (with a wavelength of approximately 157 nm). Practical use of the EUV light is being promoted as a light source.
As a shorter wavelength of light limits usable glass materials for transmitting the light, it is advantageous for the projection optical system to use reflection elements, i.e., mirrors, instead of using many refraction elements, i.e., lenses. No applicable glass materials have been proposed for the EUV light as exposure light, and a projection optical system could not include any lenses. It has thus been proposed to form a reflection type reduction projection optical system only with mirrors. In the reflection type or catoptric optical system, respective mirrors and a surface shape of each mirror are arranged axially symmetrical around one optical axis.
However, an area or slit used for exposure is a limited area that is apart from the optical axis, and one or more mirrors in the catoptric optical system receive exposure light locally or partially. Such a mirror as locally receives the exposure light generates a temperature difference in its materials, deforms its surface shape, and deteriorates its optical performance. Accordingly, Japanese Laid-Open Patent Application No. 11-243052, for example, discloses a catoptric projection optical system provided with a cooling unit for mitigating a temperature difference on the mirror.
However, the catoptric projection optical system proposed in Japanese Laid-Open Patent Application No. 11-243052 provides the cooling unit at the rear side of an unilluminated area and reflection surface of a mirror that uses only part for exposure, and thus the temperature difference generated in the mirror cannot be mitigated sufficiently. After all, the surface shape changes and deteriorates the optical performance, and this system cannot obtain desired resolution.